1. Field of the Invention
The present invention relates to an active matrix liquid crystal display, and more particularly to an active matrix liquid crystal display that makes use of a thin film two-terminal element consisting of a metal-insulator-metal structure.
2. Description of the Related Art
In recent years, applications of liquid crystal displays (LCDs) centered around those of the twisted nematic type have been expanded, and the LCDs are currently being used in large quantities in such fields as wrist watches and hand calculators. In addition, use of matrix type LCDs which make it possible to display characters, graphics and the like in an arbitrary fashion by the matrixlike arrangement of the pixels has also been introduced lately. In order to expand the applications of the LCDs, it is mandatory to increase the display capacity. Now, the rise of the voltage-transmissivity change characteristic of the conventional LCDs is not steep enough. Therefore, when the number of scanning lines for multiplexed drive is increased in order to enhance the display capacity, the ratio of the effective voltages that are applied to the selected pixels and the nonselected pixels is reduced, giving rise to crosstalks in which the transmissivity for the selected pixels is decreased while the transmissivity for the nonselected pixels is increased. As a result, the display contrast undergoes a marked decrease with the view angle for which tolerable contrast can be obtained is also narrowed, which limit the number of the scanning lines of the conventional LCDs up to around 60 lines.
For the purpose of remarkably enhancing the display capacity of the matrix type LCDs, there have been contrived active matrix LCDs in which a switching element is serially arranged to each of the pixels of the LCDs. The switching element for the trial fabrications of the active matrix LCDs disclosed so far mostly makes use of thin film transistors (TFTs) which employ amorphous silicon or polycrystalline silicon as the semiconductor material. On the other hand, active-matrix LCDs using thin-film two terminal elements (abbreviated as TFDs hereinafter) for which fabrication steps can be simplified and high yield and low cost can be expected due to relatively simple fabrication and structure are also drawing attention.
The LCD among such thin film two terminal element type active matrix LCDs (abbreviated as TFD-LCD hereinafter) which is considered to be most promising for practical application is the one which employs a metal-insulator-metal element (abbreviated as MIM element or simply MIM hereinafter) for the TFD. By connecting a TFD such as an MIM in series to the liquid crystal for each pixel, the rise in the voltage-transmissivity change characteristic of the TFD liquid crystal element can be made steep due to the high nonlinearity of the voltage-current characteristic of the TFD. This enables one to increase the number of scanning lines of the liquid crystal display.
Such a prior art example is disclosed typically in D. R. Baraff et al., "The Optimization of Metal-Insulator-Metal Nonlinear Devices for Use in Multiplexed Liquid Crystal Displays", IEEE Trans. Electron Devices, Vol. ED-28, pp. 736-739 (1981).
The most important material in MIM elements is that of the insulator layer. The most widely known insulator material is tantalum oxide (see, for example, S. Morozumi et al., "Lateral MIM-LCD with 250.times.240 Pixels," Technical Report of the Television Society of Japan (IPD 83-8), pp. 39-44, December 1983). The characteristics that are required in applying such an MIM element to a large-capacity display are that the nonlinearity coefficient .alpha. in the expression for the current (I) flowing in the element given in the form of I=aV.sup..alpha. in terms of the applied voltage (V) is large, that the current-voltage characteristic is symmetric with respect to the applied voltage regardless of its polarity, and that the capacity of the MIM element is small. Now, MIM elements using tantalum oxide have weaknesses in that the nonlinearity coefficient is in the range of 5 to 6 which is not sufficiently large, although the symmetry is satisfactory, the element capacity is large due to large permittivity, and the like. Because of this, silicon nitrides that have small permittivity have been developed as the insulator material for MIM elements (see, for example, M. Suzuki et al., "A New Active Diode Matrix LCD Using Off-Stoichiometric SiN.sub.x Layer," Proceedings of the SID, Vol. 28, pp. 101-104 (1987)).
Although the MIM elements using silicon nitrides have nonlinearity coefficient in the range of 7 to 9 which is larger than that of tantalum oxide, their voltage-current characteristic often take on asymmetric forms that depend on the polarity of the applied voltage as shown by the broken like 22 in FIG. 2. The current-voltage nonlinearity characteristic of MIM diode depends on the Poole-Frenkel carrier conduction of the bulk insulator and Schottoky barrier of metal insulator interface. This current-voltage asymmetric characteristic is due to the carrier injection difference between the bottom metal-insulator interface and the upper metal-insulater interface in the MIM structure. This carrier-injection difference is due to the formation of different kinds of modified layers (high resistance oxide layers) between the bottom and upper metal-insulator interface. As a result, when the MIM elements are utilized in liquid crystal displays, there occur flickers which bring about deterioration in the quality of pictures.